MAT03

–7–

REV. B

to bias each side of the differential pair. The 5 k

Ω collector re-

sistors noise contribution is insignificant compared to the volt-

age noise of the MAT03. Since noise in the signal path is

referred back to the input, this voltage noise is attenuated by the

gain of the circuit. Consequently, the noise contribution of the

collector load resistors is only 0.048 nV/

√Hz. This is consider-

ably less than the typical 0.8 nV/

√Hz input noise voltage of the

MAT03 transistor.

The noise contribution of the OP27 gain stages is also negligible

due to the gain in the signal path. The op amp stages amplify

the input referred noise of the transistors to increase the signal

measured with a spectrum analyzer. And, since we assume

equal noise contributions from each transistor in the MAT03,

the output is divided by

√2 to determine a single transistor’s

input noise.

Air currents cause small temperature changes that can appear

as low frequency noise. To eliminate this noise source, the

measurement circuit must be thermally isolated. Effects of extrane-

ous noise sources must also be eliminated by totally shielding

the circuit.

SUPER LOW NOISE AMPLIFIER

The circuit in Figure 14a is a super low noise amplifier with

equivalent input voltage noise of 0.32 nV/

√Hz. By paralleling

three MAT03 matched pairs, a further reduction of amplifier

noise is attained by a reduction of the base spreading resistance

by a factor of 3, and consequently the noise by

√3. Additionally,

the shot noise contribution is reduced by maintaining a high col-

lector current (2 mA/device) which reduces the dynamic emitter

resistance and decreases voltage noise. The voltage noise is in-

versely proportional to the square root of the stage current, and

current noise increases proportionally to the square root of the

stage current. Accordingly, this amplifier capitalizes on voltage

noise reduction techniques at the expense of increasing the cur-

rent noise. However, high current noise is not usually important

when dealing with low impedance sources.

Figure 14a. Super Low Noise Amplifier